Indium Antimonide (InSb) is widely recognized as being useful in magnetic field sensors, such as magnetoresistors, Hall Effect sensors (Hall sensors), and MAGFETS (magnetically sensitive field effect transistors). Indium antimonide magnetic field sensors can be used in a variety of magnetic field sensing applications, including position, speed and acceleration sensing (linear or angular), compass sensors, magnetic memory readout and control of brushless motors. While an emphasis of this invention is on automotive applications, the applicability is general.
Indium antimonide is particularly attractive for use in magnetic field sensors because it is a material that has exhibited high electron mobility in bulk form. High electron mobility is important because it provides high magnetic sensitivity. On the other hand, indium antimonide has a relatively low energy band gap, of about 0.18 electron volts at room temperature. This relatively low energy band gap gives it a relatively large intrinsic carrier density at room temperature. However, in addition, the relatively large intrinsic carrier density increases dramatically with increases in temperature. This effect tends to mask resistance changes in the indium antimonide due to changes in magnetic field.
Often such temperature-induced changes can be compensated by signal conditioning circuitry. However, with indium antimonide, the temperature-induced changes in conductivity, or resistivity, are so large that they are not readily accommodated by some of the usual types of signal conditioning circuitry. Accordingly, it is typical and preferable to stabilize the carrier density in indium antimonide with respect to temperature variations by doping it n-type, typically within a factor of about 10 of 3.times.10.sup.17 cm.sup.-3. Conventional indium antimonide magnetoresistors are usually doped in a narrower range between about 4.times.10.sup.16 cm.sup.-3 and 2.times.10.sup.17 cm.sup.-3. However, higher values may be optional for some embodiments of this invention as will be hereinafter described. Such doping increases the electron density to sufficiently higher levels that changes in intrinsic carrier density (due to temperature changes) become insubstantial compared to the dopant density, or are more accommodatable with the usual signal conditioning circuitry.
On the other hand, the resulting n-type doped material has a lower electrical resistivity, or higher conductivity, than is desired. A higher impedance device draws less current at a given voltage, and therefore dissipates less power for a given voltage bias. Alternatively, it provides a larger output voltage (change in voltage when the magnetic field changes) for a given current bias. This favors use of thin films of indium antimonide for magnetic field sensors, to obtain sensors of lower power consumption.
In the past, thin layers of indium antimonide were made by thinning down high quality bulk monocrystals. They were thinned down to layers of approximately twenty micrometers in thickness. Thinner layers were desired, for reducing series resistance of sensors made from such films. However, thinner layers were difficult to produce by thinning bulk crystals. This led to the more recent practice of growing a thin epitaxial film of indium antimonide on an insulating or semi-insulating gallium arsenide (GaAs) or indium phosphide (InP) substrate. The substrate is of high electrical resistance to minimize its non-magnetically sensitive conductivity, that is electrically in parallel with the magnetically sensitive conductivity of the epitaxial film. Such epitaxial films are typically grown to a thickness of only 2-5 micrometers. This increases film sheet resistance (film resistivity divided by thickness) by an order of magnitude over the thinned-down bulk crystal films. However, even then, such films still have had such a low sheet resistance that other techniques are used to increase total sensor resistance. For example, extensive serialization of magnetoresistor units has been used to obtain high sensor impedances, and the attendant low power dissipation.
In the case of magnetoresistors, a higher total resistance is obtained by integrating many series-connected rectangular magnetoresistor units into one sensor body. Typically, the body is an elongated mesa or line of indium antimonide. The mesa is a line of rectangular sensing units that are connected end-to-end, in series fashion. The mesa or line, i.e., the series-connected magnetoresistor rectangle units, can be as long as needed to obtain the desired total resistance. Usually, the line is sinuous or undulated in order to get the greatest length on a unit area of gallium arsenide or indium phosphide substrate material.
The individual magnetoresistor rectangles have been integrally formed in an indium antimonide elongated mesa strip, and concurrently integrally connected in series, by forming a plurality of transverse "shorting bars" along the length of the upper face of the mesa strip. The transverse "shorting bars" are ordinarily spaced a fixed distance apart, which forms a series of rectangles along the length of the mesa strip. In such a construction, net resistance in the resultant sensor is a function of the number of successive rectangles in the line.
In the past, increased sensor resistance was obtained by increasing the number of successive rectangles in the mesa or line. This, of course, increases line length, which can increase the size of the sensor in a unit area.
Alternatively, one or two electrical contacts may be added to the sides of a single rectangle and used to form a Hall effect sensor or a MAGFET sensor. A Hall effect sensor has four electrical contacts on four ends or sides of a substantially rectangular, square, or cross-shaped mesa. A MAGFET has one electrical contact on one side of a mesa and two on the opposite side. In the absence of a magnetic field, the electrical output current of a MAGFET is evenly divided between the two "opposite" contacts. In the presence of a magnetic field perpendicular to the plane of the mesa, more current flows out of one of the "opposite" contacts than through the other. In some silicon implementations of a MAGFET, an insulated gate is used to induce or control the density of current carriers between the contacts, so that the device functions as a MAGnetically sensitive Field Effect Transistor. One can make an indium antimonide "MAGFET" without the gate, by simply doping the indium antimonide film between the contacts. One can also gain additional functionality or control of an indium antimonide MAGFET by fabricating an insulated gate, as hereinafter described.
Such sensors may be used to measure the Hall effect voltage. This voltage increased with increasing magnetic field, is sensitive to the polarity of magnetic field, and is inversely proportional to the thickness of the active, or current-carrying, layer. Thus, it is desirable to make such sensors with a very thin active layer. The Hall effect voltage is also inversely proportional to the electron density in the active layer. Therefore, it is desirable to minimize the electron density in the active layer. However, the Hall voltage will be sensitive to temperature if the electron density is sensitive to temperature. Since indium antimonide and indium antimonide-rich alloys are narrow energy bandgap semiconductors, it is necessary to dope the active layer n-type in order to stabilize the electron density. Thus, many of the structures discussed here in connection with magnetoresistors are very well suited for Hall effect sensors and MAGFET sensors.
Having a large electron mobility in the active layer is also important for Hall effect sensors because it helps to reduce unintentional offset voltages which otherwise limit the minimum detectable change in magnetic field. However, the sensitivity to magnetic field in a Hall effect sensor is determined more by the areal electron density in the active layer, which needs to be minimized. Thus, it is important to make the active layer as thin and lightly doped as possible, and some compromise with reduced electron mobility is then incurred (with reduced thickness). However, one still must dope the active layer heavily enough n-type to stabilize the electron density over the operating temperature range, since otherwise the sensitivity to magnetic field varies. In view of these tradeoffs, for some applications electron mobilities of only about 30,000 cm.sup.2 V.sup.-1 s.sup.-1 at room temperature are acceptable in order to reduce the active region thickness to values which may fall somewhat below 0.25 micrometer. On the other hand, the sensitivity of a magnetoresistor to magnetic field is approximately proportional to the electron mobility squared for small magnetic fields. This places a premium on maximizing electron mobility for magnetoresistors. The optimum film parameters for MAGFETs are intermediate between those of Hall effect sensors and those of magnetoresistors. Thus, the tradeoffs between thickness, doping, and mobility are slightly different for these various sensors, but only slightly.
As indicated above, indium antimonide has been of interest as a magnetic field sensor material because it can be made with a high electron mobility. High electron mobility is obtained reproducibly only if the indium antimonide is monocrystalline, and the indium antimonide is of high quality. By "high quality" I mean that there are few crystal imperfections, such as dislocations, and the like. For example less than about 3.times.10.sup.9 cm.sup.-2 threading dislocations is desirable. The density of misfit and threading dislocations is generally much larger near an interface with a large lattice mismatch.
When growing monocrystalline thin films on a substrate, high single crystal quality is obtained if the substrate, itself, is a high quality single crystal of the same or related crystal type and has approximately the same crystal lattice constant (as the film being grown). In general, the better the crystalline match, the higher the quality of the epitaxial layer that can be grown. In addition, for highest magnetic sensitivity, and lowest power consumption, the substrate is preferably electrically insulating. To satisfy these requirements, and still other requirements, prior magnetoresistor indium antimonide epitaxial films have been typically grown on high quality monocrystalline wafers of gallium arsenide (GaAs) or indium phosphide (InP). Their large energy band gaps mean that their intrinsic carrier densities are extremely low which facilitates making them electrically insulating. Indium phosphide may be preferred because it is a closer match in lattice constant to Indium antimonide. However, gallium arsenide may be preferred because of its lower cost.
Gallium arsenide and indium phosphide are both somewhat smaller in crystal lattice size than indium antimonide. For example, the lattice constant of indium antimonide is 6.479 angstroms at room temperature. The lattice constant of indium phosphide is only 5.869 angstroms. Indium antimonide has a lattice constant that is about 10% larger. The lattice constant of gallium arsenide is 5.654 angstroms, which makes the indium antimonide lattice constant about 14% larger. This is a significant mismatch. I consider a mismatch to be significant when there is a lattice constant difference in excess of 0.07 angstrom.
As a result, the first 0.5 micrometer of an indium antimonide layer heretofore epitaxially deposited on a gallium arsenide or indium phosphide monocrystalline substrate was strained, and therefore had significant crystal imperfections. As a result, the electron mobility within that first 0.5 micrometer of deposited indium antimonide was of low electron mobility. As indicated above, this provided low magnetic sensitivity. In order to obtain higher electron mobility, and the higher magnetic sensitivity that it provided, an additional thickness was grown. This provided other disadvantages.
It should be recognized that the first 0.5 micrometer of the film is electrically in parallel with the balance of the film. Accordingly, a sufficient additional thickness had to be grown to provide a discernable magnetically variable total film resistance. Ordinarily, a total indium antimonide film thickness of at least about 2 micrometers was needed. On the other hand, if the indium antimonide thickness exceeded about 5 micrometers, the sheet resistance decreased to such an extent that size and/or power consumption of the resultant sensor became undesirable.
In this invention, I provide a magnetic field sensor having a magnetically active layer substantially of indium antimonide, and in an active layer thickness of about 0.25-0.6 micrometer. However, in this invention, this small thickness has an electron mobility approaching that often recorded for bulk crystal forms of indium antimonide. In this invention, I have been able to consistently achieve room temperature electron mobilities of 35,000-40,000 cm.sup.2 V.sup.-1 s.sup.-1, and even higher. Such a high mobility is obtained in such a small thickness because its composition is substantially pure indium antimonide, and because the small thickness is disposed on a special layer of monocrystalline indium aluminum antimonide. The special indium aluminum antimonide layer is, in turn, disposed on a monocrystalline substrate of the same crystal type. The special indium aluminum antimonide layer is of limited aluminum content and thickness, that provides ready assimilation into the indium antimonide crystal lattice, a crystal lattice closely matched with indium antimonide, and a relatively high sheet resistance. The result is an indium antimonide sensor layer of very high electron mobility and a sensor of increased magnetic sensitivity. In addition, the resulting sensor has such a high unit resistance that size of the sensor, and its attendant unit cost, can be reduced.